Methods and apparatus for generating high-density plasma

ABSTRACT

Methods and apparatus for generating a strongly-ionized plasma are described. An apparatus for generating a strongly-ionized plasma according to the present invention includes an anode and a cathode that is positioned adjacent to the anode to form a gap there between. An ionization source generates a weakly-ionized plasma proximate to the cathode. A power supply produces an electric field in the gap between the anode and the cathode. The electric field generates excited atoms in the weakly-ionized plasma and generates secondary electrons from the cathode. The secondary electrons ionize the excited atoms, thereby creating the strongly-ionized plasma.

BACKGROUND OF INVENTION

[0001] Plasma is considered the fourth state of matter. A plasma is acollection of charged particles moving in random directions. A plasmais, on average, electrically neutral. One method of generating a plasmais to drive a current through a low-pressure gas between two parallelconducting electrodes. Once certain parameters are met, the gas “breaksdown” to form the plasma. For example, a plasma can be generated byapplying a potential of several kilovolts between two parallelconducting electrodes in an inert gas atmosphere (e.g., argon) at apressure that is between about 10⁻¹ and 10⁻² Torr.

[0002] Plasma processes are widely used in many industries, such as thesemiconductor manufacturing industry. For example, plasma etching iscommonly used to etch substrate material and films deposited onsubstrates in the electronics industry. There are four basic types ofplasma etching processes that are used to remove material from surfaces:sputter etching, pure chemical etching, ion energy driven etching, andion inhibitor etching.

[0003] Plasma sputtering is a technique that is widely used fordepositing films on substrates. Sputtering is the physical ejection ofatoms from a target surface and is sometimes referred to as physicalvapor deposition (PVD). Ions, such as argon ions, are generated and thenare drawn out of the plasma, and are accelerated across a cathode darkspace. The target has a lower potential than the region in which theplasma is formed. Therefore, the target attracts positive ions. Positiveions move towards the target with a high velocity. Positive ions impactthe target and cause atoms to physically dislodge or sputter from thetarget. The sputtered atoms then propagate to a substrate where theydeposit a film of sputtered target material. The plasma is replenishedby electron-ion pairs formed by the collision of neutral molecules withsecondary electrons generated at the target surface.

[0004] Magnetron sputtering systems use magnetic fields that are shapedto trap and to concentrate secondary electrons, which are produced byion bombardment of the target surface. The plasma discharge generated bya magnetron sputtering system is located proximate to the surface of thetarget and has a high density of electrons. The high density ofelectrons causes ionization of the sputtering gas in a region that isclose to the target surface.

BRIEF DESCRIPTION OF DRAWINGS

[0005] This invention is described with particularity in the detaileddescription. The above and further advantages of this invention may bebetter understood by referring to the following description inconjunction with the accompanying drawings, in which like numeralsindicate like structural elements and features in various figures. Thedrawings are not necessarily to scale, emphasis instead being placedupon illustrating the principles of the invention.

[0006]FIG. 1 illustrates a cross-sectional view of a known plasmagenerating apparatus having a radio-frequency (RF) power supply.

[0007]FIG. 2A through FIG. 2D illustrate cross-sectional views of aplasma generating apparatus having a pulsed power supply according toone embodiment of the invention.

[0008]FIG. 3 illustrates a graphical representation of the pulse poweras a function of time for periodic pulses applied to the plasma in theplasma generating apparatus of FIG. 2A.

[0009]FIG. 4 illustrates graphical representations of the appliedvoltage, current, and power as a function of time for periodic pulsesapplied to the plasma in the plasma generating apparatus of FIG. 2A.

[0010]FIG. 5A through FIG. 5D illustrate various simulated magneticfield distributions proximate to the cathode for various electron ExBdrift currents according to the present invention.

[0011]FIG. 6A through FIG. 6D illustrate cross-sectional views ofvarious embodiments of plasma generating systems according to thepresent invention.

[0012]FIG. 7 illustrates a graphical representation of the pulse poweras a function of time for periodic pulses applied to the plasma in theplasma generating system of FIG. 6A.

[0013]FIG. 8 is a flowchart of an illustrative method of generating ahigh-density plasma according to the present invention.

DETAILED DESCRIPTION

[0014]FIG. 1 illustrates a cross-sectional view of a known plasmagenerating apparatus 100 having a radio-frequency (RF) power supply 102.The known plasma generating, apparatus 100 includes a vacuum chamber 104where the plasma 105 is generated. The vacuum chamber 104 is positionedin fluid communication with a vacuum pump 106 via a conduit 108. Thevacuum pump 106 is adapted to evacuate the vacuum chamber 104 to highvacuum. The pressure inside the vacuum chamber 104 is generally lessthan 10⁻¹ Torr. A feed gas from a feed gas source 109, such as an argongas source, is introduced into the vacuum chamber 104 through a gasinlet 110. The gas flow is controlled by a valve 112.

[0015] The plasma generating apparatus 100 also includes a cathode 114.The cathode 114 is generally in the shape of a circular disk. Thecathode 114 is electrically connected to a first terminal 118 of ablocking capacitor 120 with an electrical transmission line 122. Asecond terminal 124 of the blocking capacitor 120 is coupled to a firstoutput 126 of the RF power supply 102. An insulator 128 can be used topass the electrical transmission line 122 through a wall of the vacuumchamber 104 in order to electrically isolate the cathode 114 from thevacuum chamber 104.

[0016] An anode 130 is positioned in the vacuum chamber 104 proximate tothe cathode 114. The anode 130 is typically coupled to ground using anelectrical transmission line 132. A second output 134 of the RF powersupply 102 is also typically coupled to ground. In order to isolate theanode 130 from the vacuum chamber 104, an insulator 136 can be used topass the electrical transmission line 132 through a wall of the vacuumchamber 104. The vacuum chamber 104 can also be coupled to ground.

[0017] In operation, the RF power supply 102 applies a RF voltage pulsebetween the cathode 114 and the anode 130 that has a sufficientamplitude to ionize the argon feed gas in the vacuum chamber 104. Atypical RF driving voltage is between 100V and 1000V, and the distance138 between the cathode 114 and the anode is between about 2 cm and 10cm. Typical pressures are in the range of 10 mTorr to 100 mTorr. Typicalpower densities are in the range of 0.1W/cm² to 1W/cm². The drivingfrequency is typically 13.56 MHz. Typical plasma densities are in therange of 10⁹ cm⁻³ to 10¹¹cm⁻³, and the electron temperature is on theorder of 3 eV.

[0018] This typical ionization process is referred to as directionization or atomic ionization by electron impact and can be describedas follows:

Ar+e ⁻→Ar⁺+2e ⁻

[0019] where Ar represents a neutral argon atom in the feed gas and e⁻represents an ionizing electron generated in response to the voltageapplied between the cathode 114 and the anode 130. The collision betweenthe neutral argon atom and the ionizing electron results in an argon ion(Ar⁺) and two electrons.

[0020] The plasma discharge is maintained, at least in part, bysecondary electron emission from the cathode. However, typical operatingpressures must be relatively high so that the secondary electrons arenot lost to the anode 130 or the walls of the chamber 104. Thesepressures are not optimal for most plasma processes.

[0021] It is desirable to operate a plasma discharge at higher currentdensities, lower voltages, and lower pressures than can be obtained in aconventional glow discharge. This has led to the use of a DC magneticfield near the cathode 114 to confine the secondary electrons. Confiningthe secondary electrons substantially confines the plasma at a location(not shown) in the chamber 104 thereby increasing the plasma density atthat location for a given input power, while reducing the overall lossarea.

[0022] The magnetic confinement primarily occurs in a confinement region(not shown) where there is a relatively low magnetic field intensity.The shape and location of the confinement region depends on the designof the magnets. Generally, a higher concentration of positively chargedions in the plasma is present in the confinement region than elsewherein the chamber 104. Consequently, the uniformity of the plasma can beseverely diminished in magnetically enhanced systems.

[0023] The non-uniformity of the plasma in magnetron sputtering systemscan result in undesirable non-uniform erosion of target material andthus relatively poor target utilization. The non-uniformity of theplasma in magnetron etching systems can result in non-uniform etching ofa substrate.

[0024] Dramatically increasing the RF power applied to the plasma alonewill not result in the formation of a more uniform and denser plasma.Furthermore, the amount of applied power necessary to achieve even anincremental increase in uniformity and density can increase theprobability of generating an electrical breakdown condition leading toan undesirable electrical discharge (an electrical arc) in the chamber104.

[0025] Pulsing direct current (DC) power applied to the plasma can beadvantageous since the average discharge power can remain relatively lowwhile relatively large power pulses are periodically applied.Additionally, the duration of these large voltage pulses can be presetso as to reduce the probability of establishing an electrical breakdowncondition leading to an undesirable electrical discharge. An undesirableelectrical discharge will corrupt the plasma process and can causecontamination in the vacuum chamber 104. However, very large powerpulses can still result in undesirable electrical discharges regardlessof their duration.

[0026]FIG. 2A through FIG. 2D illustrate cross-sectional views of aplasma generating apparatus 200 having a pulsed power supply 202according to one embodiment of the invention. For example, FIG. 2Aillustrates a cross-sectional view of a plasma generating apparatus 200having a pulsed power supply 202 at a time before the pulsed powersupply 202 is activated. In one embodiment, the plasma generatingapparatus 200 includes a chamber (not shown), such as a vacuum chamberthat supports the plasma. The chamber can be coupled to a vacuum system(not shown).

[0027] The plasma generating apparatus 200 also includes a cathode 204.In one embodiment, the cathode 204 can be composed of a metal materialsuch as stainless steel or any other material that does not chemicallyreact with reactive gases. In another embodiment, the cathode 204includes a target that can be used for sputtering. The cathode 204 iscoupled to an output 206 of a matching unit 208. An input 210 of thematching unit 208 is coupled to a first output 212 of the pulsed powersupply 202. A second output 214 of the pulsed power supply 202 iscoupled to an anode 216. An insulator 218 isolates the anode 216 fromthe cathode 204.

[0028] In one embodiment, the first output 212 of the pulsed powersupply 202 is directly coupled to the cathode 204 (not shown). In oneembodiment (not shown), the second output 214 of the pulsed power supply202 is coupled to ground. In this embodiment, the anode 216 is alsocoupled to ground.

[0029] In one embodiment (not shown), the first output 212 of the pulsedpower supply 202 couples a negative voltage impulse to the cathode 204.In another embodiment (not shown), the second output 214 of the pulsedpower supply 202 couples a positive voltage impulse to the anode 216.

[0030] In one embodiment, the pulsed power supply 202 generates peakvoltage levels of up to about 30,000V. Typical operating voltages aregenerally between about 50V and 30 kV. In one embodiment, the pulsedpower supply 202 generates peak current levels of less than 1A to about5,000A or more depending on the volume of the plasma. Typical operatingcurrents varying from less than one hundred amperes to more than a fewthousand amperes depending on the volume of the plasma. In oneembodiment, the pulses generated by the pulsed power supply 202 have arepetition rate that is below 1 kHz. In one embodiment, the pulse widthof the pulses generated by the pulsed power supply 202 is substantiallybetween about one microsecond and several seconds.

[0031] The anode 216 is positioned so as to form a gap 220 between theanode 216 and the cathode 204 that is sufficient to allow current toflow through a region 222 between the anode 216 and the cathode 204. Inone embodiment, the width of the gap 220 is between approximately 0.3 cmand 10 cm. The surface area of the cathode 204 determines the volume ofthe region 222. The gap 220 and the total volume of the region 222 areparameters in the ionization process as described herein.

[0032] In one embodiment, the plasma generating apparatus 200 includes achamber (not shown), such as a vacuum chamber. The chamber is coupled influid communication to a vacuum pump (not shown) through a vacuum valve(not shown). In one embodiment, the chamber (not shown) is electricallycoupled to ground potential. One or more gas lines 224 provide feed gas226 from a feed gas source (not shown) to the chamber. In oneembodiment, the gas lines 224 are isolated from the chamber and othercomponents by insulators 228. In other embodiments, the gas lines 224can be isolated from the feed gas source using in-line insulatingcouplers (not shown). The gas source can be any feed gas 226 includingbut not limited to argon. In some embodiments, the feed gas 226 caninclude a mixture of different gases, reactive gases, or pure reactivegas gases. In some embodiments, the feed gas 226 includes a noble gas ora mixture of gases.

[0033] In operation, the feed gas 226 from the gas source is supplied tothe chamber by a gas flow control system (not shown). Preferably, thefeed gas 226 is supplied between the cathode 204 and the anode 216.Directly injecting the feed gas 226 between the cathode 204 and theanode 21 6 can increase the flow rate of the feed gas 226. This causes arapid volume exchange in the region 222 between the cathode 204 and theanode 216, which permits a high-power pulse having a longer duration tobe applied across the gap 220. The longer duration high-power pulseresults in the formation of a higher density plasma. This volumeexchange is described herein in more detail.

[0034] In one embodiment, the pulsed power supply 202 is a component inan ionization source that generates a weakly-ionized plasma 232.Referring to FIG. 2B, after the feed gas is supplied between the cathode204 and the anode 216, the pulsed power supply 202 applies a voltagepulse between the cathode 204 and the anode 216. In one embodiment, thepulsed power supply 202 applies a negative voltage pulse to the cathode204. The size and shape of the voltage pulse are chosen such that anelectric field 230 develops between the cathode 204 and the anode 216.The amplitude and shape of the electric field 230 are chosen such that aweakly-ionized plasma 232 is generated in the region 222 between theanode 216 and the cathode 204.

[0035] The weakly-ionized plasma 232 is also referred to as apre-ionized plasma. In one embodiment, the peak plasma density of thepre-ionized plasma is between about 10⁶ and 10¹² cm for argon feed gas.The pressure in the chamber can vary from about 10⁻³ to 10 Torr orhigher. The pressure can vary depending on various system parameters,such as the presence of a magnetic field proximate to the cathode 204.The peak plasma density of the weakly-ionized plasma 232 depends on theproperties of the specific plasma generating system and is a function ofthe location of the measurement in the weakly-ionized plasma 232.

[0036] In one embodiment, to generate the weakly-ionized plasma 232, thepulsed power supply 202 generates a low power pulse having an initialvoltage of between about 100V and 5 kV with a discharge current ofbetween about 0.1A and 100A. In some embodiments, the width of the pulsecan be in on the order of 0.1 microseconds up to one hundred seconds.Specific parameters of the pulse are discussed herein in more detail.

[0037] In one embodiment, prior to the generation of the weakly-ionizedplasma 232, the pulsed power supply 202 generates a potential differencebetween the cathode 204 and the anode 216 before the feed gas 226 issupplied between the cathode 204 and the anode 216. In anotherembodiment, the pulsed power supply 202 generates a current through thegap 220 after the feed gas 226 is supplied between the cathode 204 andthe anode 216.

[0038] In another embodiment, a direct current (DC) power supply (notshown) is used in an ionization source to generate and maintain theweakly-ionized or pre-ionized plasma 232. In this embodiment, the DCpower supply is adapted to generate a voltage that is large enough toignite the weakly-ionized plasma 232. In one embodiment, the DC powersupply generates an initial voltage of several kilovolts that creates aplasma discharge voltage on the order of between about 100V and 1 kVwith a discharge current in the range of about 0.1A and 100A between thecathode 204 and the anode 216 in order to generate and maintain theweakly-ionized plasma 232. The value of the discharge current depends onthe power level of the power supply and is a function of the volume ofthe weakly-ionized plasma 232. Furthermore, the presence of a magneticfield (not shown) in the region 222 can have a dramatic effect on thevalue of the applied voltage and current required to generate theweakly-ionized plasma 232.

[0039] In some embodiments (not shown), the DC power supply generates acurrent that is between about 1 mA and 100A depending on the size of theplasma generating system and the strength of a magnetic field in aregion 234. In one embodiment, before generating the weakly-ionizedplasma 232, the DC power supply is adapted to generate and maintain aninitial peak voltage between the cathode 204 and the anode 216 beforethe introduction of the feed gas 226.

[0040] In another embodiment, an alternating current (AC) power supply(not shown) is used to generate and maintain the weakly-ionized orpre-ionized plasma 232. For example, the weakly-ionized or pre-ionizedplasma 232 can be generated and maintained using electron cyclotronresonance (ECR), capacitively coupled plasma discharge (CCP), orinductively coupled plasma (ICP) discharge.

[0041] AC power supplies can require less power to generate and maintaina weakly-ionized plasma than a DC power supply. In addition, thepre-ionized or weakly-ionized plasma 232 can be generated by numerousother techniques, such as UV radiation techniques, X-ray techniques,electron beam techniques, ion beam techniques, or ionizing filamenttechniques. These techniques include components used in ionizationsources according to the invention. In some embodiments, theweakly-ionized plasma is formed outside of the region 222 and thendiffuses into the region 222.

[0042] Forming the weakly-ionized or pre-ionized plasma 232substantially eliminates the probability of establishing a breakdowncondition in the chamber when high-power pulses are applied between thecathode 204 and the anode 216. The probability of establishing abreakdown condition is substantially eliminated because theweakly-ionized plasma 232 has a low-level of ionization that provideselectrical conductivity through the plasma. This conductivitysubstantially prevents the setup of a breakdown condition, even whenhigh power is applied to the plasma.

[0043] In one embodiment, as the feed gas 226 is pushed through theregion 222, the weakly-ionized plasma 232 diffuses somewhathomogeneously through the region 234. This homogeneous diffusion tendsto facilitate the creation of a highly uniform strongly-ionized plasmain the region 234.

[0044] In one embodiment (not shown), the weakly-ionized plasma 232 canbe trapped proximate to the cathode 204 by a magnetic field.Specifically, electrons in the weakly-ionized plasma 232 can be trappedby a magnetic field generated proximate to the cathode 204. In oneembodiment, the strength of the magnetic field is between about fiftyand two thousand gauss.

[0045] In one embodiment, a magnet assembly (not shown) generates themagnet field located proximate to the cathode 204. The magnet assemblycan include permanent magnets (not shown), or alternatively,electro-magnets (not shown). The configuration of the magnet assemblycan be varied depending on the desired shape and strength of themagnetic field. In alternate embodiments, the magnet assembly can haveeither a balanced or unbalanced configuration. In one embodiment, themagnet assembly includes switching electromagnets, which generate apulsed magnetic field proximate to the cathode 204. In some embodiments,additional magnet assemblies (not shown) can be placed at variouslocations around and throughout the chamber (not shown).

[0046] Referring to FIG. 2C, once the weakly-ionized plasma 232 isformed, the pulsed power supply 202 generates high-power pulses betweenthe cathode 204 and the anode 216 (FIG. 2C). The desired power level ofthe high-power pulses depends on several factors including the densityof the weakly-ionized plasma 232, and the volume of the plasma, forexample. In one embodiment, the power level of the high-power pulse isin the range of about 1 kW to about 10 MW or higher.

[0047] Each of the high-power pulses is maintained for a predeterminedtime that, in some embodiments, is approximately one microsecond to tenseconds. The repetition frequency or repetition rate of the high-powerpulses, in some embodiments, is in the range of between about 0.1 Hz to1 kHz. The average power generated by the pulsed power supply 202 can beless than one megawatt depending on the volume of the plasma. In oneembodiment, the thermal energy in the cathode 204 and/or the anode 216is conducted away or dissipated by liquid or gas cooling such as heliumcooling (not shown).

[0048] The high-power pulses generate a strong electric field 236between the cathode 204 and the anode 216. The strong electric field 236is substantially located in the region 222 between the cathode 204 andthe anode 216. In one embodiment, the electric field 236 is a pulsedelectric field. In another embodiment, the electric field 236 is aquasi-static electric field. By quasi-static electric field we mean anelectric field that has a characteristic time of electric fieldvariation that is much greater than the collision time for electronswith neutral gas particles. Such a time of electric field variation canbe on the order of ten seconds. The strength and the position of thestrong electric field 236 will be discussed in more detail herein.

[0049] Referring to FIG. 2D, the high-power pulses generate ahighly-ionized or a strongly-ionized plasma 238 from the weakly-ionizedplasma 232 (FIG. 2C). The strongly-ionized plasma 238 is also referredto as a high-density plasma. The discharge current that is formed fromthe strongly-ionized plasma 238 can be on the order of about 5 kA ormore with a discharge voltage in the range of between about 50V and 500Vfor a pressure that is on the order of between about 100 mTorr and 10Torr.

[0050] In one embodiment, the strongly-ionized plasma 238 tends todiffuse homogenously in the region 234. The homogenous diffusion createsa more homogeneous plasma volume. Homogenous diffusion is described inmore detail with reference to FIG. 5A through FIG. 5D.

[0051] Homogeneous diffusion is advantageous for many plasma processes.For example, plasma etching processes having homogenous diffusionaccelerate ions in the strongly-ionized plasma 238 towards the surfaceof the substrate (not shown) being etched in a more uniform manner thanwith conventional plasma etching. Consequently, the surface of thesubstrate is etched more uniformly. Plasma processes having homogeneousdiffusion can achieve high uniformity without the necessity of rotatingthe substrate.

[0052] Also, magnetron sputtering systems having homogenous diffusionaccelerate ions in the strongly-ionized plasma 238 towards the surfaceof the sputtering target in a more uniform manner than with conventionalmagnetron sputtering. Consequently, the target material is depositedmore uniformly on a substrate without the necessity of rotating thesubstrate and/or the magnetron. Also, the surface of the sputteringtarget is eroded more evenly and, thus higher target utilization isachieved.

[0053] In one embodiment, the plasma generating apparatus 200 of thepresent invention generates a relatively high electron temperatureplasma and a relatively high-density plasma. One application for thestrongly-ionized plasma 238 of the present invention is ionized physicalvapor deposition (IPVD) (not shown), which is a technique that convertsneutral sputtered atoms into positive ions to enhance a sputteringprocess.

[0054] Referring again to FIG. 2D, the strong electric field 236facilitates a multi-step ionization process of the feed gas 226 thatsubstantially increases the rate at which the strongly-ionized plasma238 is formed. In one embodiment, the feed gas is a molecular gas andthe strong electric field 236 enhances the formation of ions in theplasma. The multi-step or stepwise ionization process is described asfollows.

[0055] A pre-ionizing voltage is applied between the cathode 204 and theanode 216 across the feed gas 226 to form the weakly-ionized plasma 232.The weakly-ionized plasma 232 is generally formed in the region 222 anddiffuses to the region 234 as the feed gas 226 continues to flow. In oneembodiment (not shown) a magnetic field is generated in the region 222and extends to the center of the cathode 204. This magnetic field tendsto assist in diffusing electrons from the region 222 to the region 234.The electrons in the weakly-ionized plasma 232 are substantially trappedin the region 234 by the magnetic field. In one embodiment, the volumeof weakly-ionized plasma in the region 222 is rapidly exchanged with anew volume of feed gas 226.

[0056] After the formation of the weakly-ionized plasma 232 (FIG. 2C),the pulsed power supply 202 applies a high-power pulse between thecathode 204 and the anode 216. This high-power pulse generates thestrong electric field 236 in the region 222 between the cathode 204 andthe anode 216. The strong electric field 236 results in collisionsoccurring between neutral atoms 240, electrons (not shown), and ions 242in the weakly-ionized plasma 232. These collisions generate numerousexcited atoms 244 in the weakly-ionized plasma 232.

[0057] The accumulation of excited atoms 244 in the weakly-ionizedplasma 232 alters the ionization process. In one embodiment, the strongelectric field 236 facilitates a multi-step ionization process of anatomic feed gas that significantly increases the rate at which thestrongly-ionized plasma 238 is formed. The multi-step ionization processhas an efficiency that increases as the density of excited atoms 244 inthe weakly-ionized plasma 232 increases. The strong electric field 236enhances the formation of ions of a molecular or atomic feed gas.

[0058] In one embodiment, the distance or gap 220 between the cathode204 and the anode 216 is chosen so as to maximize the rate of excitationof the atoms. The value of the electric field 236 in the region 222depends on the voltage level applied by the pulsed power supply 202 andthe size of the gap 220 between the anode 216 and the cathode 204. Insome embodiments, the strength of the electric field 236 can varybetween about 2V/cm and 10⁵ V/cm depending on various system parametersand operating conditions of the plasma system.

[0059] In some embodiments, the gap 220 can be between about 0.30 cm andabout 10 cm depending on various parameters of the desired plasma. Inone embodiment, the electric field 236 in the region 222 is rapidlyapplied to the pre-ionized or weakly-ionized plasma 232. In someembodiments, the rapidly applied electric field 236 is generated byapplying a voltage pulse having a rise time that is between about 0.1microsecond and ten seconds.

[0060] In one embodiment, the dimensions of the gap 220 and theparameters of the applied electric field 236 are varied in order todetermine the optimum condition for a relatively high rate of excitationof the atoms 240 in the region 222. For example, an argon atom requiresan energy of about 11.55 eV to become excited. Thus, as the feed gas 226flows through the region 222, the weakly-ionized plasma 232 is formedand the atoms 240 in the weakly-ionized plasma 232 experience a stepwiseionization process.

[0061] The excited atoms 244 in the weakly-ionized plasma 232 thenencounter the electrons (not shown) that are in the region 234. Theexcited atoms 244 only require about 4 eV of energy to ionize whileneutral atoms 240 require about 15.76 eV of energy to ionize. Therefore,the excited atoms 244 will ionize at a much higher rate than the neutralatoms 240. In one embodiment, ions 242 in the strongly-ionized plasma238 strike the cathode 204 causing secondary electron emission from thecathode 204. These secondary electrons interact with neutral 240 orexcited atoms 244 in the strongly-ionized plasma 238. This processfurther increases the density of ions 242 in the strongly-ionized plasma238 as the feed gas 226 is exchanged.

[0062] The multi-step ionization process corresponding to the rapidapplication of the electric field 236 can be described as follows:

Ar+e ⁻→Ar^(*) +e ⁻

Ar^(*) +e ⁻→Ar⁺+2e ⁻

[0063] where Ar represents a neutral argon atom 240 in the feed gas 226and e⁻ represents an ionizing electron generated in response to apre-ionized plasma 232, when sufficient voltage is applied between thecathode 204 and the anode 216. Additionally, Ar^(*) represents anexcited argon atom 244 in the weakly-ionized plasma 232. The collisionbetween the excited argon atom 244 and the ionizing electron results inan argon ion (Ar⁺) and two electrons.

[0064] The excited argon atoms 244 generally require less energy tobecome ionized than neutral argon atoms 240. Thus, the excited atoms 244tend to more rapidly ionize near the surface of the cathode 204 than theneutral argon atoms 240. As the density of the excited atoms 244 in theplasma increases, the efficiency of the ionization process rapidlyincreases. This increased efficiency eventually results in anavalanche-like increase in the density of the strongly-ionized plasma238. Under appropriate excitation conditions, the proportion of theenergy applied to the weakly-ionized plasma 232, which is transformed tothe excited atoms 244, is very high for a pulsed discharge in the feedgas 226.

[0065] Thus, in one aspect of the invention, high-power pulses areapplied to a weakly-ionized plasma 232 across the gap 220 to generatethe strong electric field 236 between the anode 216 and the cathode 204.This strong electric field 236 generates excited atoms 244 in theweakly-ionized plasma 232. The excited atoms 244 are rapidly ionized byinteractions with the secondary electrons that are emitted by thecathode 204. The rapid ionization results in a strongly-ionized plasma238 having a large ion density being formed in the area 234 proximate tothe cathode 204. The strongly-ionized plasma 238 is also referred to asa high-density plasma.

[0066] In one embodiment of the invention, higher density plasma isgenerated by controlling the flow of the feed gas 226 in the region 222.In this embodiment, a first volume of feed gas 226 is supplied to theregion 222. The first volume of feed gas 226 is then ionized to form aweakly-ionized plasma 232 in the region 222. Next, the pulsed powersupply 202 applies a high-power electrical pulse across theweakly-ionized plasma 232. The high-power electrical pulse generates astrongly-ionized plasma 238 from the weakly-ionized plasma 232.

[0067] The level and duration of the high-power electrical pulse islimited by the level and duration of the power that the strongly-ionizedplasma 238 can absorb before the high-power discharge contracts andterminates. In one embodiment, the strength and the duration of thehigh-power electrical pulse are increased and thus the density of thestrongly-ionized plasma 238 is increased by increasing the flow rate ofthe feed gas 226.

[0068] In one embodiment, the strongly-ionized plasma 238 is transportedthrough the region 222 by a rapid volume exchange of feed gas 226. Asthe feed gas 226 moves through the region 222, it interacts with themoving strongly-ionized plasma 238 and. also becomes strongly-ionizedfrom the applied high-power electrical pulse. The ionization process canbe a combination of direct ionization and/or stepwise ionization asdescribed herein. Transporting the strongly-ionized plasma 238 throughthe region 222 by a rapid volume exchange of the feed gas 226 increasesthe level and the duration of the power that can be applied to thestrongly-ionized plasma 238 and, thus, generates a higher densitystrongly-ionized plasma in the region 234.

[0069] The efficiency of the ionization process can be increased byapplying a magnetic field (not shown) proximate to the cathode 204, asdescribed herein. The magnetic field tends to trap electrons in theweakly-ionized plasma 232 and also tends to trap secondary the electronsproximate to the cathode 204. The trapped electrons ionize the excitedatoms 244 generating the strongly-ionized plasma 238. In one embodiment,the magnetic field is generated in the region 222 to substantially trapelectrons in the area where the weakly-ionized plasma 232 is ignited.

[0070] In one embodiment, the plasma generating system 200 can beconfigured for plasma etching. In another embodiment, the plasmagenerating system 200 can be configured for plasma sputtering. Inparticular, the plasma generating system 200 can be used for sputteringmagnetic materials. Known magnetron sputtering systems generally are notsuitable for sputtering magnetic materials because the magnetic fieldgenerated by the magnetron can be absorbed by the magnetic targetmaterial. RF diode sputtering is sometimes used to sputter magneticmaterials. However, RF diode sputtering generally has poor filmthickness uniformity and relatively low deposition rates.

[0071] The plasma generating system 200 can be adapted to sputtermagnetic materials by including a target assembly having a magnetictarget material and by driving that target assembly with an RF powersupply (not shown). For example, an RF power supply can provide an RFpower that is on order of about 10 kW. A substantially uniformweakly-ionized plasma can be generated by applying RF power across afeed gas that is located proximate to the target assembly. Thestrongly-ionized plasma is generated by applying a strong electric fieldacross the weakly-ionized plasma as described herein. The RF powersupply applies a negative voltage bias to the target assembly. Ions inthe strongly-ionized plasma bombard the target material thereby causingsputtering.

[0072] The plasma generating system 200 can also be adapted to sputterdielectric materials. Dielectric materials can be sputtered by driving atarget assembly including a dielectric target material with an RF powersupply. (not shown). For example, an RF power supply can provide an RFpower that is on order of about 10 kW. A substantially uniformweakly-ionized plasma can be generated by applying RF power across afeed gas that is located proximate to the target assembly.

[0073] In another embodiment, a DC power supply (not shown) is used tocreate a weakly-ionized plasma 232 according to the present invention.In this embodiment, the dielectric target material is positionedrelative to the cathode 204 such that an area of the cathode 204 canconduct a direct current between the anode 216 and the cathode 204.

[0074] In one embodiment, a magnetic field is generated proximate to thetarget assembly in order to trap electrons in the weakly-ionized plasma.The strongly-ionized plasma is generated by applying a strong electricfield across the weakly-ionized plasma as described herein. The RF powersupply applies a negative voltage bias to the target assembly. Ions inthe strongly-ionized plasma bombard the target material thereby causingsputtering.

[0075] In one embodiment, a strongly-ionized plasma 238 according to thepresent invention is used to generate an ion beam. An ion beam sourceaccording to the present invention includes the plasma generatingapparatus described herein and an additional electrode (not shown) thatis used to accelerate ions in the plasma. In one embodiment, theexternal electrode is a grid. The ion beam source according to thepresent invention can generate a very high-density ion flux. Forexample, the ion beam source can generate ozone flux. Ozone is a highlyreactive oxidizing agent that can be used for many applications such ascleaning process chambers, deodorizing air, purifying water, andtreating industrial wastes.

[0076]FIG. 3 illustrates a graphical representation 300 of the appliedpower of a pulse as a function of time for periodic pulses applied tothe plasma in the plasma generating apparatus 200 of FIG. 2A. At timet₀, the feed gas 226 flows between the cathode 204 and the anode 216before the pulsed power supply 202 is activated. The time required for asufficient quantity of gas 226 to flow between the cathode 204 and theanode 216 depends on several factors including the flow rate of the gas226 and the desired pressure in the region 222.

[0077] In one embodiment (not shown), the pulsed power supply 202 isactivated before the feed gas 226 flows into the region 222. In thisembodiment, the feed gas 226 is injected between the anode 216 and thecathode 204 where it is ignited by the pulsed power supply 202 togenerate the weakly-ionized plasma 232.

[0078] In one embodiment, the feed gas 226 flows between the anode 216and the cathode 204 between time t₀ and time t₁. At time t₁, the pulsedpower supply 202 generates a pulse 302 between the cathode 204 and theanode 216 that has a power between about 0.01 kW and 100 kW depending onthe volume of the plasma. The pulse 302 is sufficient to ignite the feedgas 226 to generate the weakly-ionized plasma 232.

[0079] In one embodiment (not shown), the pulsed power supply 202applies a potential between the cathode 204 and the anode 216 before thefeed gas 226 is delivered into the region 222. In this embodiment, thefeed gas 226 is ignited as it flows between the cathode 204 and theanode 216. In other embodiments, the pulsed power supply 202 generatesthe pulse 302 between the cathode 204 and the anode 216 during or afterthe feed gas 226 is delivered into the region 222.

[0080] The power generated by the pulsed power supply 202 partiallyionizes the feed gas 226 that is located in the region 222 between thecathode 204 and the anode 216. The partially ionized gas is alsoreferred to as a weakly-ionized plasma or a pre-ionized plasma 232 (FIG.2B). The formation of the weakly-ionized plasma 232 substantiallyeliminates the possibility of creating a breakdown condition whenhigh-power pulses are applied to the weakly-ionized plasma 232 asdescribed herein.

[0081] In one embodiment, the power is continuously applied for betweenabout one microsecond and one hundred seconds to allow the pre-ionizedplasma 232 to form and to be maintained at a sufficient plasma density.In one embodiment, the power from the pulsed power supply 202 iscontinuously applied after the weakly-ionized plasma 232 is ignited inorder to maintain the weakly-ionized plasma 232. The pulsed power supply202 can be designed so as to output a continuous nominal power in orderto generate and sustain the weakly-ionized plasma 232 until a high-powerpulse is delivered by the pulsed power supply 202.

[0082] At time t₂, the pulsed power supply 202 delivers a high-powerpulse 304 across the weakly-ionized plasma 232. In some embodiments, thehigh-power pulse 304 has a power that is in the range of between about 1kW and 10 MW depending on parameters of the plasma generating apparatus200. The high-power pulse has a leading edge 306 having a rise time ofbetween about 0.1 microseconds and ten seconds.

[0083] The high-power pulse 304 has a power and a pulse width that issufficient to transform the weakly-ionized plasma 232 to astrongly-ionized plasma 238 (FIG. 2D). The strongly-ionized plasma 238is also referred to as a high-density plasma. In one embodiment, thehigh-power pulse 304 is applied for a time that is in the range ofbetween about ten microseconds and ten seconds. At time t₄, thehigh-power pulse 304 is terminated.

[0084] The power supply 202 maintains the weakly-ionized plasma 232after the delivery of the high-power pulse 304 by applying backgroundpower that, in one embodiment, is between about 0.01 kW and 100 kW. Thebackground power can be a pulsed or continuously applied power thatmaintains the pre-ionization condition in the plasma, while the pulsedpower supply 202 prepares to deliver another high-power pulse 308.

[0085] At time t₅, the pulsed power supply 202 delivers anotherhigh-power pulse 308. The repetition rate between the high-power pulses304, 308 is, in one embodiment, between about 0.1 Hz and 1 kHz. Theparticular size, shape, width, and frequency of the high-power pulses304, 308 depend on various factors including process parameters, thedesign of the pulsed power supply 202, the design of the plasmagenerating apparatus 200, the volume of the plasma, and the pressure inthe chamber. The shape and duration of the leading edge 306 and thetrailing edge 310 of the high-power pulse 304 is chosen to sustain theweakly-ionized plasma 232 while controlling the rate of ionization ofthe strongly-ionized plasma 238.

[0086] In one embodiment, the particular size, shape, width, andfrequency of the high-power pulse 304 is chosen to control the densityof the strongly-ionized plasma 238. In one embodiment, the particularsize, shape, width, and frequency of the high-power pulse 304 is chosento control the etch rate of a substrate (not shown). In one embodiment,the particular size, shape, width, and frequency of the high-power pulse304 is chosen to control the rate of sputtering of a sputtering target(not shown).

[0087]FIG. 4 illustrates graphical representations 320, 322, and 324 ofthe absolute value of applied voltage, current, and power, respectively,as a function of time for periodic pulses applied to the plasma in theplasma generating apparatus 200 of FIG. 2A. In one embodiment, at timet₀ (not shown), the feed gas 226 flows proximate to the cathode 204before the pulsed power supply 202 is activated. The time required for asufficient quantity of feed gas 226 to flow proximate to the cathode 204depends on several factors including the flow rate of the feed gas 226and the desired pressure in the region 222.

[0088] In the embodiment shown in FIG. 4, the power supply 202 generatesa constant power. At time t ,the pulsed power supply 202 generates avoltage 326 across the anode 216 and the cathode 204. In one embodiment,the voltage 326 is approximately between 100V and 5 kV. The periodbetween time t₀ and time t₁ (not shown) can be on the order of severalmicroseconds up to several milliseconds. At time t₁, the current 328 andthe power 330 have constant value.

[0089] Between time t₁ and time t₂, the voltage 326, the current 328,and the power 330 remain constant as the weakly-ionized plasma 232 (FIG.2B) is generated. The voltage 332 at time t₂ is between about 100V and 5kV. The current 334 at time t₂ is between about 0.1A and 10A. The power336 delivered at time t₂ is between about 0.01 kW and 100 kW.

[0090] The power 336 generated by the pulsed power supply 202 partiallyionizes the gas 226 that is located in the region 222 between thecathode 204 and the anode 216. The partially ionized gas is alsoreferred to as a weakly-ionized plasma or a pre-ionized plasma 232. Asdescribed herein, the formation of weakly-ionized plasma 232substantially eliminates the possibility of creating a breakdowncondition when high-power pulses are applied to the weakly-ionizedplasma 232. The suppression of this breakdown condition substantiallyeliminates the occurrence of undesirable arcing between the anode 216and the cathode 204.

[0091] In one embodiment, the period between time t₁ and time t₂ isbetween about one microsecond and one hundred seconds to allow thepre-ionized plasma 232 to form and be maintained at a sufficient plasmadensity. In one embodiment, the power 336 from the pulsed power supply202 is continuously applied to maintain the weakly-ionized plasma 232.The pulsed power supply 202 can be designed so as to output a continuousnominal power into order to sustain the weakly-ionized plasma 232.

[0092] Between time t₂ and time t₃, the pulsed power supply 202 deliversa large voltage pulse 338 across the weakly-ionized plasma 232. In someembodiments, the large voltage pulse 338 has a voltage that is in therange of 200V to 30 kV. In some embodiments, the period between time t₂and time t₃ is between about 0.1 microseconds and ten seconds. The largevoltage pulse 338 is applied between time t₃ and time t₄, before thecurrent across the weakly-ionized plasma 232 begins to increase. In oneembodiment, the period between time t₃ and time t₄ can be between aboutten nanoseconds and one microsecond.

[0093] Between time t₄ and time t₅, the voltage 340 drops as the current342 increases. The power 344 also increases between time t₄ and time t₅,until a quasi-stationary state exists between the voltage 346 and thecurrent 348. The period between time t₄ and time t₅ can be on the orderof hundreds of nanoseconds.

[0094] In one embodiment, at time t₅, the voltage 346 is between about50V and 30 kV thousand volts, the current 348 is between about 10A and 5kA and the power 350 is between about 1 kW and 10 MW. The power 350 iscontinuously applied to the plasma until time t₆. In one embodiment, theperiod between time t₅ and time t₆ is approximately between onemicrosecond and ten seconds.

[0095] The pulsed power supply 202 delivers a high-power pulse having amaximum power 350 and a pulse width that is sufficient to transform theweakly-ionized plasma 232 to a strongly-ionized plasma 238 (FIG. 2D). Attime t₆ the maximum power 350 is terminated. In one embodiment, thepulsed power supply 202 continues to supply a background power that issufficient to maintain the plasma after time t₆.

[0096] In one embodiment, the power supply 202 maintains the plasmaafter the delivery of the high-power pulse by continuing to apply apower 352 that can be between about 0.01 kW and 100 kW to the plasma.The continuously generated power maintains the pre-ionization conditionin the plasma, while the pulsed power supply 202 prepares to deliver thenext high-power pulse.

[0097] At time t₇, the pulsed power supply 202 delivers the nexthigh-power pulse (not shown). In one embodiment, the repetition ratebetween the high-power pulses is between about 0.1 Hz and 1 kHz. Theparticular size, shape, width, and frequency of the high-power-pulsesdepend on various factors including process parameters, the design ofthe pulsed power supply 202, the design of the plasma generating system200, the volume of plasma, the density of the strongly-ionized plasma238, and the pressure in the region 222.

[0098] In another embodiment (not shown), the power supply 202 generatesa constant voltage. In this embodiment, the applied voltage 320 iscontinuously applied from time t₂ until time t₆. The current 322 and thepower 324 rise until time t₆ in order to maintain a constant voltagelevel, and then the voltage 320 is terminated. The parameters of thecurrent, power and voltage are optimized for generating exited atoms.

[0099] In one embodiment of the invention, the efficiency of theionization process is increased by generating a magnetic field proximateto the cathode 204. The magnetic field tends to trap electrons in theweakly-ionized plasma 232 proximate to the cathode 204. The trappedelectrons ionize the excited atoms 244 thereby generating thestrongly-ionized plasma 238. In this embodiment, magnetically enhancedplasma has strong diamagnetic properties. The term “strong diamagneticproperties” means that the magnetically enhanced high-density plasmadischarge tends to exclude external magnetic fields from the plasmavolume.

[0100]FIG. 5A through FIG. 5D illustrate various simulated magneticfield distributions 400, 402, 404, and 406 proximate to the cathode 204for various electron ExB drift currents in a magnetically enhancedplasma generating apparatus according to one embodiment of theinvention. The magnetically enhanced plasma generating apparatusincludes a magnet assembly 407 that is positioned proximate to thecathode 204. The magnet assembly 407 generates a magnetic fieldproximate to the cathode 204. In one embodiment, the strength of themagnetic field is between about fifty and two thousand gauss. Thesimulated magnetic fields distributions 400, 402, 404, and 406 indicatethat high-power plasmas having high current density tend to diffusehomogeneously in an area 234′ of the magnetically enhanced plasmagenerating apparatus.

[0101] The high-power pulses between the cathode 204 and the anode 216generate secondary electrons from the cathode 204 that move in asubstantially circular motion proximate to the cathode 204 according tocrossed electric and magnetic fields. The substantially circular motionof the electrons generates an electron ExB drift current. The magnitudeof the electron ExB drift current is proportional to the magnitude ofthe discharge current in the plasma and, in one embodiment, isapproximately in the range of between about three and ten times themagnitude of the discharge current.

[0102] In one embodiment, the substantially circular electron ExB driftcurrent generates a magnetic field that interacts with the magneticfield generated by the magnet assembly 407. In one embodiment, themagnetic field generated by the electron ExB drift current has adirection that is substantially opposite to the magnetic field generatedby the magnet assembly 407. The magnitude of the magnetic fieldgenerated by the electron ExB drift current increases with increasedelectron ExB drift current. The homogeneous diffusion of thestrongly-ionized plasma in the region 234′ is caused, at least in part,by the interaction of the magnetic field generated by the magnetassembly 407 and the magnetic field generated by the electron ExB driftcurrent.

[0103] In one embodiment, the electron ExB drift current defines asubstantially circular shape for low current density plasma. However, asthe current density of the plasma increases, the substantially circularelectron ExB drift current tends to have a more complex shape as theinteraction of the magnetic field generated by the magnet assembly 407,the electric field generated by the high-power pulse, and the magneticfield generated by the electron ExB drift current becomes more acute.For example, in one embodiment, the electron ExB drift current has asubstantially cycloidal shape. The exact shape of the electron ExB driftcurrent can be quite elaborate and depends on various factors.

[0104] For example, FIG. 5A illustrates the magnetic field lines 408produced from the interaction of the magnetic field generated by themagnet assembly 407 and the magnetic field generated by an electron ExBdrift current 410 illustrated by a substantially circularly shaped ring.The electron ExB drift current 410 is generated proximate to the cathode204.

[0105] In the example shown in FIG. 5A, the electron ExB drift current410 is approximately 100A. In one embodiment of the invention, theelectron ExB drift current 410 is between approximately three and tentimes as great as the discharge current. Thus, in the example shown inFIG. 5A, the discharge current is approximately between 10A and 30A. Themagnetic field lines 408 shown in FIG. 5A indicate that the magneticfield generated by the magnet assembly 407 is substantially undisturbedby the relatively small magnetic field that is generated by therelatively small electron ExB drift current 410.

[0106]FIG. 5B illustrates the magnetic field lines 412 produced from theinteraction of the magnetic field generated by the magnet assembly 407and the magnetic field generated by an electron ExB drift current 414.The electron ExB drift current 414 is generated proximate to the cathode204. In the example shown in FIG. 5B, the electron ExB drift current 414is approximately 300A. Since the electron ExB drift current 414 istypically between about three and ten times as great as the dischargecurrent, the discharge current in this example is approximately between30A and 100A.

[0107] The magnetic field lines 412 that are generated by the magnetassembly 407 are substantially undisturbed by the relatively smallmagnetic field generated by the relatively small electron ExB driftcurrent 414. However, the magnetic field lines 416 that are closest tothe electron ExB drift current 414 are somewhat distorted by themagnetic field generated by the electron ExB drift current 414. Thedistortion suggests that a larger electron ExB drift current shouldgenerate a stronger magnetic field that will interact more strongly withthe magnetic field generated by the magnet assembly 407.

[0108]FIG. 5C illustrates the magnetic field lines 418 that are producedfrom the interaction of the magnetic field generated by the magnetassembly 407 and the magnetic field generated by an electron ExB driftcurrent 420. The electron ExB drift current 420 is generated proximateto the cathode 204. In the example shown in FIG. 5C, the electron ExBdrift current 420 is approximately 1,000A. Since the electron ExB driftcurrent 420 is typically between about three and ten times as great asthe discharge current, the discharge current in this example isapproximately between 10A and 300A.

[0109] The magnetic field lines 418 that are generated by the magnetassembly 407 exhibit substantial distortion that is caused by therelatively strong magnetic field generated by the relatively largeelectron ExB drift current 420. Thus, the larger electron ExB driftcurrent 420 generates a stronger magnetic field that strongly interactswith and can begin to dominate the magnetic field generated by themagnet assembly 407.

[0110] The interaction of the magnetic field generated by the magnetassembly 407 and the magnetic field generated by the electron ExB driftcurrent 420 generates magnetic field lines 422 that are somewhat moreparallel to the surface of the cathode 204 than the magnetic field lines408, 412, and 416 in FIG. 5A and FIG. 5B. The magnetic field lines 422allow the strongly-ionized plasma 238 to more uniformly distributeitself in the area 234′. Thus, the strongly-ionized plasma 238 issubstantially uniformly diffused in the area 234′.

[0111]FIG. 5D illustrates the magnetic field lines 424 produced from theinteraction of the magnetic field generated by the magnet assembly 407and the magnetic field generated by an electron ExB drift current 426.The electron ExB drift current 426 is generated proximate to the cathode204. In the example shown in FIG. 5D, the electron ExB drift current 426is approximately 5 kA. The discharge current in this example isapproximately between 500A and 1,700A.

[0112] The magnetic field lines 424 generated by the magnet assembly 407are relatively distorted due to their interaction with the relativelystrong magnetic field generated by the relatively large electron ExBdrift current 426. Thus, in this embodiment, the relatively largeelectron ExB drift current 426 generates a very strong magnetic fieldthat is stronger than the magnetic field generated by the magnetassembly 407.

[0113]FIG. 6A through FIG. 6D illustrate cross-sectional views ofalternative embodiments of plasma generating systems 200′, 200″, 200′″and 200″″, according to the present invention. The plasma generatingsystem 200′ of FIG. 6A includes an electrode 452 that generates aweakly-ionized or pre-ionized plasma. The electrode 452 is also referredto as a pre-ionizing filament electrode and is a component in anionization source that generates the weakly-ionized plasma.

[0114] In one embodiment, the electrode 452 is coupled to an output 454of a power supply 456. The power supply 456 can be a DC power supply oran AC power supply. An insulator 458 isolates the electrode 452 from theanode 216. In one embodiment, the electrode 452 is substantially shapedin the form of a ring electrode. In other embodiments, the electrode 452is substantially shaped in a linear form or any other shape that issuitable for pre-ionizing the plasma.

[0115] In one embodiment, a second output 460 of the power supply 456 iscoupled to the cathode 204. The insulator 218 isolates the cathode 204from the anode 216. In one embodiment, the power supply 456 generates anaverage output power that is in the range of between about 0.01 kW and100 kW. Such an output power is sufficient to generate a suitablecurrent between the electrode 452 and the cathode 204 to pre-ionize feedgas 226 that is located proximate to the electrode 452.

[0116] In operation, the plasma generating apparatus 200′ functions in asimilar manner to the plasma generating apparatus 200 of FIG. 2A, butwith some operational differences. In one embodiment (not shown) amagnetic field is generated proximate to the cathode 204. In oneembodiment, the strength of the magnetic field is between about fiftyand two thousand gauss. The feed gas 226 is supplied proximate to theelectrode 452 and the cathode 204.

[0117] The power supply 456 applies a suitable current between thecathode 204 and the electrode 452. The parameters of the current arechosen to establish a weakly-ionized plasma in the region 234 proximateto the electrode 452. In one embodiment, the power supply 456 generatesa voltage of between about 100V and 5 kV with a discharge current thatis between about 0.1A and 100A depending on the size of the system. Anexample of specific parameters of the voltage will be discussed hereinin more detail in connection with FIG. 7.

[0118] In one embodiment, the resulting pre-ionized plasma density is inthe range between approximately 10⁶ and 10 cm⁻³ for argon sputteringgas. In one embodiment, the pressure in the region 234 is in the rangeof approximately 10⁻³ to 10 Torr or higher. The pressure can varydepending on various system parameters, such as the presence of amagnetic field proximate to the cathode 204. As previously discussed,the weakly-ionized or pre-ionized plasma substantially eliminates thepossibility of establishing a breakdown condition between the cathode204 and the anode 216 when high-power pulses are applied to the plasma.

[0119] The pulsed power supply 202 then generates a high-power pulsebetween the cathode 204 and the anode 216. The high-power pulsegenerates a strongly-ionized plasma from the weakly-ionized plasma. Theparameters of the high-power pulse depend on various parametersincluding the volume of the plasma, the desired plasma density, and thepressure in the region 234.

[0120] In one embodiment, the high-power pulse between the cathode 204and the anode 216 is in the range of about 1 kW to about 10 MW. In oneembodiment, the discharge current density that can be generated from thestrongly-ionized plasma is greater than about 1A/cm² for a pressure ofapproximately 10 mTorr.

[0121] In one embodiment, the high-power pulse has a pulse width that isin the range of approximately one microsecond to several seconds. In oneembodiment, the repetition rate of the high-power discharge is in therange of between about 0.1 Hz to 10 kHz. In one embodiment, the averagepower generated by the pulsed power supply is less than 1 MW dependingon the size of the system. In one embodiment, the thermal energy in thecathode 204 and/or the anode 216 can be conducted away or dissipated byliquid or gas cooling (not shown).

[0122] In one embodiment (not shown), a magnetic field is generatedproximate to the cathode 204. The strongly-ionized plasma tends todiffuse homogenously in the area 234 due to the interaction of generatedmagnetic fields, as described herein in connection with FIG. 5A thoughFIG. 5D.

[0123]FIG. 6B is a cross-sectional view of another embodiment of aplasma generating apparatus 200″ according to the present invention.This embodiment is similar to the plasma generating apparatus 200′ ofFIG. 6A. However, in this embodiment, the electrode 452′, which is acomponent of the ionization source, substantially surrounds the cathode204. The position of the electrode 452′ relative to the cathode 204 ischosen to achieve particular electrical conditions in the gap 220between the anode 216 and the cathode 204.

[0124] For example, a distance 462 between the electrode 452′ and thecathode 204 can be varied by changing the diameter of the electrode452′. In one embodiment, the distance 462 can be varied from about 0.1cm to about 10 cm. The distance 462 can be optimized to generate asustainable weakly-ionized plasma in the region 234. The verticalposition of the electrode 452′ relative to the cathode 204 can also bevaried.

[0125] The pre-ionizing electrode 452′ is not physically located in theregion 222 between the anode 216 and the cathode 204. Therefore, thepre-ionizing electrode 452′ does not interfere with the strong electricfield that results when a high-power pulse from the pulsed power supply202 is applied between the anode 216 and the cathode 204. Additionally,the location of the pre-ionizing electrode 452′ results in a moreuniformly distributed weakly-ionized plasma in the region 234.

[0126] In operation, the power supply 456 applies a voltage between thecathode 204 and the electrode 452′. The voltage generates aweakly-ionized or pre-ionized plasma proximate to the electrode 452′ andthe cathode 204. The pre-ionized plasma substantially eliminates thepossibility of establishing a breakdown condition when high-power pulsesfrom the pulsed power supply 202 are applied to the plasma.

[0127] In one embodiment, the power supply 456 is a DC power supply thatgenerates a DC voltage that is in the range of between about 100V and 5kV with a discharge current that is in the range of between about 0.1Aand 100A. In another embodiment, the power supply 456 is an AC powersupply that generates voltage pulses between the cathode 204 and theelectrode 452′.

[0128]FIG. 6C is a cross-sectional view of another embodiment of aplasma generating apparatus 200′″ according to the present invention.The configuration of the electrode 452 and the cathode 204′ can affectthe parameters of the electric field generated between the electrode 452and the cathode 204′. The parameters of the electric field can influencethe ignition of the pre-ionized plasma as well as the pre-ionizationprocess generally. This embodiment creates the necessary conditions forbreakdown of the feed gas and ignition of the weakly-ionized plasma inthe region 222 between the anode 216 and the cathode 204′.

[0129] In the embodiment illustrated by FIG. 6C, the electric fieldlines (not shown) generated between the cathode 204′ and the electrode452 are substantially perpendicular to the cathode 204′ at the point 470on the cathode 204′. The electric field in the gap 472 between theelectrode 452 and the cathode 204′ is adapted to ignite the plasma fromthe feed gas 226 flowing through the gap 472. The efficiency of thepre-ionization process can be increased using this embodiment dependingupon parameters, such as the magnetic field strength and the pressure inthe area proximate to the cathode 204′.

[0130]FIG. 6D is a cross-sectional view of another embodiment of aplasma generating apparatus 200″″ according to the present invention. Inthis embodiment, the electric field lines (not shown) generated betweenthe cathode 204″ and the electrode 452 are substantially perpendicularto the cathode 204″ at the point 474. The electric field in the gap 476between the electrode 452 and the cathode 204″ is adapted to ignite theplasma from the feed gas 226 flowing through the gap 476. The efficiencyof the pre-ionization process can be increased using this embodimentdepending upon parameters, such as the magnetic field strength and thepressure in the area proximate to the cathode 204″.

[0131]FIG. 7 illustrates a graphical representation 500 of the pulsepower as a function of time for periodic pulses applied to the plasma inthe plasma generating system 200′ of FIG. 6A. In one embodiment, thefeed gas 226 flows in the region 222 proximate to the electrode 452 attime t₀. before either the power supply 456 or the pulsed power supply202 are activated.

[0132] In another embodiment, the power supply 456 and/or the pulsedpower supply 202 are activated at time t₀ before the gas 226 flows inthe region 222 proximate to the electrode 452. In this embodiment, thefeed gas 226 is injected between the electrode 452 and the cathode 204where it is ignited by the power supply 456 to generate theweakly-ionized plasma.

[0133] The time required for a sufficient quantity of gas 226 to flowinto the region 222 depends on several factors including the flow rateof the gas 226 and the desired operating pressure. At time t₁, the powersupply 456 generates a power 502 that is in the range of between about0.01 kW to about 100 kW between the electrode 452 and the cathode 204.The power 502 causes the gas 226 proximate to the electrode 452 tobecome partially ionized, thereby generating a weakly-ionized plasma ora pre-ionized plasma.

[0134] At time t₂, the pulsed power supply 202 delivers a high-powerpulse 504 to the weakly-ionized plasma that is on the order of less than1 kW to about 10 MW depending on the volume of the plasma and theoperating pressure. The high-power pulse 504 is sufficient to transformthe weakly-ionized plasma to a strongly-ionized plasma. The high-powerpulse has a leading edge 506 having a rise time that is between about0.1 microseconds and ten seconds.

[0135] In one embodiment, the pulse width of the high-power pulse 504 isin the range of between about one microsecond and ten seconds. Thehigh-power pulse 504 is terminated at time t₄. Even after the deliveryof the high-power pulse 504, the power 502 from the power supply 456 iscontinuously applied to sustain the pre-ionized plasma, while the pulsedpower supply 202 prepares to deliver another high-power pulse 508. Inanother embodiment (not shown), the power supply 456 is an AC powersupply and delivers suitable power pulses to ignite and sustain theweakly-ionized plasma.

[0136] At time t₅, the pulsed power supply 202 delivers anotherhigh-power pulse 508. In one embodiment, the repetition rate of thehigh-power pulses can be between about 0.1 Hz and 10 kHz. The particularsize, shape, width, and frequency of the high-power pulse depend on theprocess parameters, such as the operating pressure, the design of thepulsed power supply 202, the presence of a magnetic field proximate tothe cathode 204, and the volume of the plasma. The shape and duration ofthe leading edge 506 and the trailing edge 510 of the high-power pulse504 is chosen to control the rate of ionization of the strongly-ionizedplasma.

[0137]FIG. 8 is a flowchart 600 of an illustrative process of generatinga high-density or strongly-ionized plasma according to the presentinvention. The process is initiated (step 602) by activating varioussystems in the plasma generating apparatus 200 of FIG. 2A. For example,a chamber (not shown) is initially pumped down to a specific pressure(step 604). Next, the pressure in the chamber is evaluated (step 606).In one embodiment, feed gas 226 is then pumped into the chamber (step608). The gas pressure is evaluated (step 610). If the gas pressure iscorrect, the pressure in the chamber is again evaluated (step 612).

[0138] An appropriate magnetic field is generated proximate to the feedgas 226 (not shown) when the pressure in the chamber is correct. In oneembodiment, a magnet assembly (not shown) can include at least onepermanent magnet, and thus the magnetic field is generated constantly,even before the process is initiated. In another embodiment, a magneticassembly (not shown) includes at least one electromagnet, and thus themagnetic field is generated only when the electromagnet is operating.

[0139] The feed gas 226 is ionized to generate a weakly-ionized plasma232 (step 614). In one embodiment, the weakly-ionized plasma 232 can begenerated by creating a relatively low current discharge in the gap 220between the cathode 204 and the anode 216 of FIG. 2A. In anotherembodiment, the weakly-ionized plasma 232 can be generated by creating arelatively low current discharge between the electrode 452 and thecathode 204 of FIG. 6A. In yet another embodiment (not shown), anelectrode is heated to emit electrons proximate to the cathode 204. Inthis embodiment, a relatively low current discharge is created betweenthe anode 216 and the electrode in order to generate the weakly-ionizedplasma 232.

[0140] In the embodiment shown in FIG. 2A, the weakly-ionized plasma 232is generated by applying a potential across the gap 220 between thecathode 204 and the anode 216 before the introduction of the feed gas226. In the embodiment shown in FIG. 6A, the weakly-ionized plasma 232is generated by applying a potential difference between the electrode452 and the cathode 204 before the introduction of the feed gas 226 togenerate the weakly-ionized plasma 232.

[0141] After the gas is weakly-ionized (step 616), a strongly-ionizedplasma 238 (FIG. 2D) is generated from the weakly-ionized plasma 232(step 618). In one embodiment, the strongly-ionized plasma 238 isgenerated by applying a high-power pulse between the cathode 204 and theanode 216. The high-power pulse results in a strong electric field 236being generated in the gap 220 between the anode 216 and the cathode 204as described herein. The strong electric field 236 results in a stepwiseionization process of the feed gas 226. In one embodiment, moleculargases are used for the feed gas 226. In this embodiment, the strongelectric field 236 increases the formation of ions, which enhances thestrongly-ionized plasma 238. In one embodiment, the strongly-ionizedplasma 238 is substantially homogeneous in the area 234 of FIG. 2D.

[0142] After the strongly-ionized plasma 238 is formed (step 620), it ismaintained as required by the plasma process (step 622). Once the plasmaprocess is completed (step 624), the plasma process is ended (step 626).

[0143] Equivalents

[0144] While the invention has been particularly shown and describedwith reference to specific preferred embodiments, it should beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the spirit and scopeof the invention as defined herein.

What is claimed is:
 1. An apparatus for generating a strongly-ionizedplasma, the apparatus comprising: an ionization source that generates aweakly-ionized plasma from a volume of feed gas; a power supply thatapplies an electrical pulse across the weakly-ionized plasma to generatethe strongly-ionized plasma; and a means for exchanging thestrongly-ionized plasma with a second volume of feed gas while applyingthe electrical pulse across the second volume of feed gas to generate anadditional strongly-ionized plasma.
 2. The apparatus of claim 1 whereinthe power supply applies the electrical pulse across the weakly-ionizedplasma to excite atoms in the weakly-ionized plasma and to generatesecondary electrons, the secondary electrons ionizing the excited atoms,thereby creating the strongly-ionized plasma.
 3. The apparatus of claim1 further comprising a gas exchange means for exchanging theweakly-ionized plasma with a third volume of feed gas while applying theelectrical pulse across the third volume of feed gas.
 4. The apparatusof claim 1 wherein the power supply generates a constant power.
 5. Theapparatus of claim 1 wherein the power supply generates a constantvoltage.
 6. The apparatus of claim 1 wherein the ionization source ischosen from the group comprising an electrode coupled to a DC powersupply, an electrode coupled to an AC power supply, a UV source, anX-ray source, an electron beam source, an ion beam source, aninductively coupled plasma source, a capacitively coupled plasma source,and a microwave plasma source.
 7. The apparatus of claim 1 furthercomprising a magnet that is positioned to generate a magnetic fieldproximate to the weakly-ionized plasma, the magnetic field trappingelectrons in the weakly-ionized plasma.
 8. The apparatus of claim 7wherein the magnet comprises an electromagnet.
 9. The apparatus of claim7 wherein the magnet is movable.
 10. A method for generating astrongly-ionized plasma, the method comprising: ionizing a volume offeed gas to form a weakly-ionized plasma; applying an electrical pulseacross the weakly-ionized plasma to generate the strongly-ionizedplasma; and exchanging the strongly-ionized plasma with a second volumeof feed gas while applying the electrical pulse across the second volumeof feed gas to generate an additional strongly-ionized plasma.
 11. Themethod of claim 10 wherein the applying the electrical pulse across theweakly-ionized plasma excites atoms in the weakly-ionized plasma andgenerates secondary electrons, the secondary electrons ionizing theexcited atoms, thereby creating a strongly-ionized plasma.
 12. Themethod of claim 10 further comprising exchanging the weakly-ionizedplasma with a third volume of feed gas while applying the electricalpulse across the third volume of feed gas.
 13. The method of claim 10wherein the applying the. electrical pulse comprises applying aquasi-static electric field across the weakly-ionized plasma.
 14. Themethod of claim 10 further comprising selecting at least one of a pulseamplitude and a pulse width of the electrical pulse in order to increasean ionization rate of the strongly-ionized plasma.
 15. The method ofclaim 10 further comprising selecting at least one of a pulse amplitudeand a pulse width of the electrical pulse in order to cause thestrongly-ionized plasma to be substantially uniform.
 16. The method ofclaim 10 wherein the electrical pulse comprises a rise time that isbetween about 0.1 microsecond and 10 seconds.
 17. The method of claim 10wherein the peak plasma density of the weakly-ionized plasma is lessthan about 10 cm
 18. The method of claim 10 wherein the peak plasmadensity of the strongly-ionized plasma is greater than about 10¹² cm⁻³.19. The method of claim 10 wherein the ionizing the feed gas comprisesexposing the feed gas to one of a static electric field, an pulsedelectric field, UV radiation, X-ray radiation, electron beam radiation,and an ion beam.
 20. The method of claim 10 further comprisinggenerating a magnetic field proximate to the weakly-ionized plasma, themagnetic field trapping electrons in the weakly-ionized plasma.
 21. Themethod of claim 10 wherein the weakly-ionized plasma reduces theprobability of developing an electrical breakdown condition.
 22. Anapparatus for generating a strongly-ionized plasma, the apparatuscomprising: an anode; a cathode that is positioned adjacent to the anodeand forming a gap there between; an ionization source that generates aweakly-ionized plasma proximate to the cathode; and a power supply thatproduces an electric field across the gap, the electric field generatingexcited atoms in the weakly-ionized plasma and generating secondaryelectrons from the cathode, the secondary electrons ionizing the excitedatoms, thereby creating the strongly-ionized plasma.
 23. The apparatusof claim 22 wherein the power supply generates a constant power.
 24. Theapparatus of claim 22 wherein the power supply generates a constantvoltage.
 25. The apparatus of claim 22 wherein the electric fieldcomprises a quasi-static electric field.
 26. The apparatus of claim 22wherein the electric field comprises a pulsed electric field.
 27. Theapparatus of claim 22 wherein a rise time of the electric field ischosen to increase an ionization rate of the excited atoms in theweakly-ionized plasma.
 28. The apparatus of claim 22 wherein theweakly-ionized plasma reduces the probability of developing anelectrical breakdown condition between the anode and the cathode. 29.The apparatus of claim 22 wherein the strongly-ionized plasma issubstantially uniform proximate to the cathode.
 30. The apparatus ofclaim 22 wherein a dimension of the gap between the anode and thecathode is chosen to increase an ionization rate of the excited atoms inthe weakly-ionized plasma.
 31. The apparatus of claim 22 wherein theionization source is chosen from the group comprising an electrodecoupled to a DC power supply, an electrode coupled to an AC powersupply, a UV source, an X-ray source, an electron beam source, an ionbeam source, an inductively coupled plasma source, a capacitivelycoupled plasma source, and a microwave plasma source.
 32. The apparatusof claim 22 further comprising a magnet that is positioned to generate amagnetic field proximate to the weakly-ionized plasma, the magneticfield trapping electrons in the weakly-ionized plasma proximate to thecathode.
 33. A method for generating a strongly-ionized plasma, themethod comprising: ionizing a feed gas to generate a weakly-ionizedplasma proximate to a cathode; and applying an electric field across theweakly-ionized plasma in order to excite atoms in the weakly-ionizedplasma and to generate secondary electrons from the cathode, thesecondary electrons ionizing the excited atoms, thereby creating thestrongly-ionized plasma.
 34. The method of claim 33 wherein the applyingthe electric field comprises applying a quasi-static electric field. 35.The method of claim 33 wherein the applying an electric field comprisesapplying the electric field at a constant power.
 36. The method of claim33 wherein the applying an electric field comprises applying theelectric field at a constant voltage.
 37. The method of claim 33 whereinthe applying the electric field comprises applying an electrical pulseacross the weakly-ionized plasma.
 38. The method of claim 37 furthercomprising selecting at least one of a pulse amplitude and a pulse widthof the electrical pulse in order to increase an ionization rate of thestrongly-ionized plasma.
 39. The method of claim 37 further comprisingselecting at least one of a pulse amplitude and a pulse width of theelectrical pulse in order to cause the strongly-ionized plasma to besubstantially uniform in an area adjacent to a surface of the cathode.40. The method of claim 33 wherein the strongly-ionized plasma issubstantially uniform proximate to the cathode.
 41. The method of claim33 further comprising generating a magnetic field proximate to theweakly-ionized plasma, the magnetic field trapping electrons in theweakly-ionized plasma.
 42. The method of claim 33 wherein theweakly-ionized plasma reduces the probability of developing anelectrical breakdown condition between the anode and the cathode.
 43. Anapparatus for generating a strongly-ionized plasma, the apparatuscomprising: means for ionizing a volume of feed gas to form aweakly-ionized plasma; means for applying an electrical pulse across theweakly-ionized plasma to generate the strongly-ionized plasma; and meansfor exchanging the strongly-ionized plasma with a second volume of feedgas while applying the electrical pulse across the second volume of feedgas to generate an additional strongly-ionized plasma.
 44. An apparatusfor generating a strongly-ionized plasma, the apparatus comprising:means for ionizing a feed gas to generate a weakly-ionized plasmaproximate to a cathode; and means for applying an electric field acrossthe weakly-ionized plasma in order to excite atoms in the weakly-ionizedplasma and to generate secondary electrons from the cathode, thesecondary electrons ionizing the excited atoms, thereby creating thestrongly-ionized plasma.